Ships and barges are usually built on land and then launched by sliding them down an inclined plane longitudinally or sideways. Other structures are not readily launched in this way because of their size and shape so they are largely built in water to make launching unnecessary. Some of the large concrete offshore structures used in North Sea oil exploration and production are of this type. However, it is usually less expensive and faster to build vessels and offshore structures wholly on land than to build them partially or wholly on water.
An alternative approach is to construct a graving dock, build the ship, barge or other structure within the graving dock and then flood the graving dock to float the vessel, barge or structure. Removal or opening of the dike or gate to the sea permits movement of the vessel or structure out to sea. While this approach is practical it is often very expensive, largely because of the cost of constructing the graving dock and its dike or gate.
There are many areas on docks or dockyards where large size nonhoistable ships, barges and floatable offshore structures can be built. However, the dock surfaces are generally 5 to 30 feet above the sea or water level so that it is not possible to simply push them over the dock edge to launch them into the water. Similarly, delaunching a ship, barge or floating structure from water onto a dock is not readily accomplished when they are too big to hoist onto the dock. A need accordingly exists for apparatus and methods of launching and delaunching ships, barges and floatable offshore structures from and to such docks.